Ata' bin Abdullah al-Khurasani heard Sa'id bin al-Musayyib (may Allah have mercy on him) say: Whoever resolves to stay four nights while he is a traveller should pray the complete prayer. Imam Malik (may Allah have mercy on him) said: This is the position the people of knowledge have always held.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5455